Increased concentrations of endogenous digitalis-like sodium pump inhibitors have been observed in chronic hypertension in humans and laboratory animals, but whether they are a cause or effect of the hypertension remains to be clarified. Digitalis-like factors might constitute a previously unrecognized group of stress hormones that could contribute to the development of human hypertension via interaction with dietary sodium intake. The present research tests the hypothesis that the blood pressure elevations occurring in micropigs during behavioral stress are mediated by increases in circulating levels of endogenous digitalis-like factors and inhibition of erythrocyte sodium pump activity.